Airfoil-magnetic hybrid bearing and a control system thereof

ABSTRACT

An airfoil-magnetic hybrid bearing, which is a combination of a magnetic bearing and airfoil bearing includes a housing, an airfoil bearing, a magnetic bearing, coils, and a coupling segment. The housing is around a rotating shaft of a rotation device. The airfoil bearing is positioned between the housing and the rotating shaft to form dynamic pressure to levitate the rotating shaft during rotation of the rotating shaft. The magnetic bearing includes cores positioned radially between inner surface of the housing and outer surface of the airfoil bearing. The cores are projected to the center of the rotating shaft. The coils wound to the cores to levitate the rotating shaft using a magnetic attractive force generated by a current application thereto. The coupling segment is configured to couple the airfoil bearing to the magnetic bearing directly.

TECHNICAL FIELD

The present invention relates to a bearing which supports the rotatingshaft of a high speed rotation device and a control system thereof, andmore particularly to an airfoil-magnetic hybrid bearing being acombination of a magnetic bearing and airfoil bearing for supporting arotating body of the rotation device, and a control system thereof. Thehigh speed rotation devices using the hybrid bearing of the presentinvention can be an industrial compressor, a fan, an auxiliary powerunit for aircraft, air conditioning system, etc.

BACKGROUND ART

There are many kinds of bearings for rotation devices. An airfoilbearing generates small friction force by making an air film between therotating shaft and housing. Thus, the airfoil bearing is used for highspeed rotation device but it has the dis-advantage of being difficult tocontrol. However, a magnetic bearing can adjust the magnetic force ofthe electromagnet. Thus it has the advantage of being easy to control.However, it is also difficult to control at high speeds. In particular,if the rotating shaft is tilted to one side during high speeds andcontacts the magnetic pole, then the system may be damaged. Thus, ahybrid bearing having an airfoil bearing and magnetic bearing usedtogether has been developed for taking all the advantages of the airfoilbearing and the magnetic bearing.

Referring to FIG. 1, a prior art airfoil-magnetic hybrid bearing for ahigh speed rotation device will be described in detail.

FIG. 1 is a side sectional view illustrating an exemplary prior artairfoil-magnetic bearing. As illustrated, the prior art airfoil-magnetichybrid bearing includes an airfoil bearing (30) around the rotatingshaft (20) disposed at the center portion of a cylindrical housing (10)and a magnetic bearing (40) disposed around the airfoil bearing (30).

The housing (10) has cores (41) formed parallel to the rotating shaft(20) and coils (42) wound around the cores (41) respectively. Thehousing extends more deeply than the magnetic bearing (40) and theairfoil bearing (30) is mounted on the extended portion of the housing.

However, the core (41) of the prior art airfoil-magnetic hybrid bearingmust be disposed parallel to the longitudinal direction of the housing(10), while the coil (42) must be wound around the core (41). Thus, thetotal length of the rotation rotor becomes longer. The airfoil bearing(30) is not coupled to the magnetic bearing (40) directly. An axiallength (L1) of the housing (10) becomes longer since the housing (10)has the airfoil bearing (30) and the magnetic bearing (40) together.

Further, a radial thickness of the housing (10) around the rotatingshaft (20) becomes larger since there is a gap between the airfoilbearing (30) and the magnetic bearing (40). Thus, total volume of thehybrid bearing becomes larger.

Further, the magnetic bearing (40) is positioned away from the rotatingshaft due to the gap between the airfoil bearing (30) and the magneticbearing (40). Thus, the magnetic bearing (40) has a low supportingefficiency.

DISCLOSURE OF INVENTION Technical Problem

The present invention is conceived in order to solve the above problems.It is an objective of the present invention to provide anairfoil-magnetic hybrid bearing having a magnetic bearing being directlycoupled to an airfoil bearing.

Further, it is another objective of the present invention to provide anairfoil-magnetic hybrid bearing for controlling the magnetic bearing tobe used before levitation speed, during external force application, andin resonant frequency area.

Technical Solution

The present invention provides an airfoil-magnetic hybrid bearingcomprising: a housing around a rotating shaft of a rotation device; anairfoil bearing positioned between the housing and the rotating shaftfor forming dynamic pressure to levitate the rotating shaft duringrotation of the rotating shaft; a magnetic bearing including a pluralityof cores positioned radially between an inner surface of the housing andouter surface of the airfoil bearing, the cores being projected to thecenter of the rotating shaft, and coils wound to the cores to levitatethe rotating shaft using a magnetic attractive force generated by acurrent application thereto; and a coupling segment for coupling theairfoil bearing to the magnetic bearing directly.

It is desired that the magnetic bearing operates in resonant speed areaof a vibration mode without the magnetic bearing operation. Also, itshould operate between the initial drive of the rotating shaft andlevitation speed of the rotating shaft, the airfoil bearing levitatingthe rotating shaft at the levitation speed. Further, it should operatewhen the vibration displacement of the rotating shaft is bigger than thepredetermined amplitude.

Preferably, the spaces between the cores and the coils are charged bydielectric bodies. A fixing slot is formed at one of the dielectricbodies. One end of the airfoil bearing is coupled to the couplingsegment. Preferably, the coupling segment is inserted into the fixingslot in the axial direction for fastening.

Preferably, the fixing slot becomes narrower as it gets closer to thecenter of the rotating shaft. The coupling segment has a shapecorresponding to the fixing slot.

The airfoil bearing includes a porous foil formed by a porous elasticbody and a top foil laminated to the porous foil adjacent to therotating shaft. One end of the airfoil bearing is fixed to the couplingsegment and the other end is free.

Further, the present invention provides a control system for anairfoil-magnetic hybrid bearing comprising: an airfoil-magnetic hybridbearing including a housing around the outer surface of a rotating shaftof a rotation device, an airfoil bearing positioned between the housingand the rotating shaft for generating dynamic pressure to levitate andsupport the rotating shaft during the rotation of the rotating shaft,and a magnetic bearing positioned between the airfoil bearing and thehousing to levitate the rotating shaft using a magnetic attractiveforce; a position sensor sensing the vibration amplitude of the centerof the rotating shaft; a speed sensor sensing the rotating speed of therotating shaft; and a controller controlling the magnetic bearing. Thecontroller operates the magnetic bearing when the rotating speed of therotating shaft sensed by the speed sensor is under levitation speed,when the rotating speed of the rotating shaft is in the resonant area inthe vibration mode without operation of the magnetic bearing, and whenthe displacement of the rotating shaft sensed by the position sensorexceeds the predetermined vibration amplitude. The airfoil bearing canlevitate the rotating shaft at the levitation speed.

The controller operates the magnetic bearing between the start andlevitation speed during the rotation device drive. After the end of therotation device drive, the controller operates the magnetic bearingbetween the levitation speed and zero speed, the airfoil bearinglevitating the rotating shaft at the levitation speed.

The resonant area in the vibration mode without operation of themagnetic bearing is from 90% to 110% of the resonant speed.

Advantageous Effects

According to the present invention, a hybrid structure of an airfoilbearing and the magnetic bearing can reduce the length of the rotor.Thus, it can drive close to the rigid mode area and the vibration can bereduced. Further, the total volume of the system can be reduced.

Moreover, according to the present invention, the airfoil bearing can beremoved easily by using the coupling segment. Thus, the airfoil bearingcan be replaced easily in instances where the airfoil bearing wasbroken, or depending on the circumstance, for changing the thickness ormaterial of the airfoil bearing.

Moreover, according to the present invention, the magnetic bearing isdriven in initial condition (under the levitation speed), in a conditionof resonant area, and in a condition with external force application.Thus, the magnetic bearing can levitate the rotating shaft in thevarious states. Further, the rotating shaft can be supported efficientlyand its constant position can be controlled. Further, the durability ofthe bearing can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side sectional view illustrating an example of a prior artairfoil-magnetic hybrid bearing.

FIG. 2 is a front sectional view illustrating an embodiment of anairfoil-magnetic hybrid bearing according to the present invention.

FIG. 3 is a side sectional view of FIG. 2 comparing it to the prior art.

FIG. 4 is a front sectional view illustrating a magnetic bearing and ahousing of the airfoil-magnetic hybrid bearing of FIG. 2.

FIG. 5 is a front sectional view illustrating an airfoil bearing and acoupling segment of the airfoil-magnetic hybrid bearing of FIG. 2.

FIG. 6 is a perspective view illustrating a process for coupling theairfoil bearing to the magnetic bearing.

FIG. 7 is a schematic view illustrating a magnetic field generated bycurrent application to the magnetic bearing of FIG. 5.

FIG. 8 is a block diagram illustrating a control system of anairfoil-magnetic hybrid bearing according to the present invention.

FIG. 9 is a flow chart illustrating a control process of the magneticbearing of the control system of FIG. 8.

FIG. 10 illustrates orbit graphs of the rotating shafts, one rotatingshaft being supported by the airfoil-magnetic hybrid bearing while theother being supported by only an airfoil bearing.

FIG. 11 is a graph illustrating controlled amplitude in resonant areausing the airfoil-magnetic hybrid bearing according to the presentinvention.

FIG. 12 illustrates waterfall graphs near the free ends of the rotatingshafts of a prior art airfoil bearing and an airfoil-magnetic hybridbearing according to the present invention.

FIG. 13 shows waterfall graphs near the turbine of the rotating shaft ofthe prior art airfoil bearing of FIG. 12 and the airfoil-magnetic hybridbearing according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, referring to FIGS. 2 to 6, an embodiment ofairfoil-magnetic hybrid bearing according to the present invention willbe described in detail.

FIG. 2 is a front sectional view illustrating an embodiment of anairfoil-magnetic hybrid bearing according to the present invention. FIG.3 is a side sectional view of FIG. 2.

As shown in FIG. 2, the present invention comprises a magnetic bearing(101) having a plurality of cores (111) positioned radially from insideof a housing projecting to a center of the housing (110), and whereinthe coils (120) are wound to the cores (111), and dielectric bodies(112) charged therebetween; an airfoil bearing (102) having porous foil(130) and top foil (140); and a coupling segment (150) coupling themagnetic bearing (101) to the airfoil bearing (102).

A gap between adjacent cores (111) becomes narrower at the portioncloser to the center of the housing. Thus, winding density of theoutside portion (121) of the coil (120) is different to that of theinside portion (122). Each coil is wound continuously without cutting.Coil turns of the outside portion is equal to those of the insideportion.

At one of the dielectric bodies (112) of the magnetic bearing (101), thedielectric substance is not charged in some areas of the center portion.A groove for receiving the coupling segment (150) is formed where thedielectric substance is not charged.

One end of the airfoil bearing (102) is fixed to the coupling segment(150). As illustrated, the porous foil (130) and the top foil (140) arefixed to the coupling segment (150) by a screw (160). The rotating shaft(103) or a portion facing the magnetic bearing of the rotating shaft ismade of ferromagnetic material, to thereby be worked by a magnetic forceof the magnetic bearing (101).

The porous foil (130) of the airfoil bearing (102) is a wired meshedfoil made of porous elastic material. And the top foil (140) ispositioned adjacent to the rotating shaft (not shown) for supporting therotating shaft by an air film formed between the top foil and therotating shaft during the rotation of the rotating shaft. The porousfoil (140) has high damping performance. Thus, it is suitable as anairfoil bearing element of a super high speed rotation device.

As shown in FIG. 3 (b), if the direction of a magnetic field is changed,then the inner end of each core (111) will be as close as possible tothe rotating shaft (103), maximizing efficiency of the magnetic bearing(101).

However, in FIG. 3 (b), the direction of the magnetic field isillustrated by marks ⊚ and □. Mark ⊚ means a direction coming from thedrawing, and Mark □ means a direction going into the drawing. Thedirection of the magnetic field is perpendicular to the direction of themagnetic field shown in FIG. 3( a) of the prior art. Thus, in prior art(a), cores are disposed parallel to a rotating shaft. However, in thepresent invention (b), the cores (111) are disposed perpendicular to therotating shaft. The housing (110) need not be formed around the magneticbearing (101) and the airfoil bearing (102) wholly. Thus, the axiallength (L2) of the magnetic bearing is much shorter than the length (L1)of the magnetic bearing (101) for reducing the space occupied axially bythe bearing shown in prior art (a).

FIG. 4 is a front sectional view illustrating a magnetic bearing and ahousing of the airfoil-magnetic hybrid bearing of FIG. 2. And FIG. 5 isa front sectional view illustrating an airfoil bearing and a couplingsegment of the airfoil-magnetic hybrid bearing of FIG. 2. As shown inFIG. 4, the dielectric substance is not charged at the center portion ofthe dielectric bodies (112) of the magnetic bearing (101) for making apentagonal fixing slot (113). As shown in FIG. 5, the shape of thecoupling segment (150) corresponds to the fixing slot (113). One end ofthe porous foil (130) and the top foil (140) is fixed to the couplingsegment (150) by a screw (160).

Hereinafter, functions and effects of the above airfoil-magnetic hybridbearing will be described.

The fixing slot (113) becomes narrower at portions nearer to the centerof the housing (110). The shape of the coupling segment (150)corresponds to that of the fixing slot (113). Thus, it becomes narrowerat portions nearer to the center. Moreover, the coupling segment (150)cannot escape in the radial direction. And, the airfoil bearing (102) iscoupled to the magnetic bearing (101) by the coupling segment (150)while it is not coupled to the housing. This reduces the total bearingvolume.

Also, the airfoil bearing (102) blocks the core and the rotating shaft.Thus, during magnetic bearing (101) operation, the rotating shaft, whichis a ferromagnetic body, cannot contact the magnetic bearing (101)directly.

FIG. 6 is a perspective view illustrating a process for coupling theairfoil bearing to the magnetic bearing. As illustrated, one end of theairfoil bearing (102) is coupled to the coupling segment (150) while thecoupling segment (150) is inserted into the fixing slot (113) axiallyfor easy coupling between the magnetic bearing (101) and the airfoilbearing (102). Consequently, damaged airfoil bearing (102) can bereplaced easily. Also, when the thickness or material of the airfoilbearing needs to be changed, the airfoil bearing (102) can be easilyreplaced.

Referring FIG. 7 to 9, an embodiment of a control system forairfoil-magnetic hybrid bearing according to the present invention willnow be described.

FIG. 7 is a schematic view illustrating a magnetic field generated bycurrent application to the magnetic bearing of FIG. 5. As illustrated,eight cores (111) are vertically symmetric in this embodiment. Magneticfields generated by coils (112) wound to two adjacent cores (111) are inthe same direction. However, magnetic fields generated by coil (112)wound to the following two adjacent cores (111) are in the reverseddirection.

With current application, four circular magnetic fields are formedaround the rotating shaft. Upper attracting forces generated in themagnetic fields should be stronger considering the mass of the rotatingshaft. Thus, current (I₀₁+i), adding control current (i) to bias current(I₀₁), is applied to each coil generating the upper portion of themagnetic field while currents (I₀₁−i), subtracting control current (i)from bias current (I₀₁), is applied to each coil generating the lowerportion of the magnetic field.

FIG. 8 is a block diagram illustrating a control system of anairfoil-magnetic hybrid bearing according to the present invention. Asillustrated, the control system includes a sensor (211) for rotating thespeed of the rotating shaft (103), a vibration analyzer (212) mounted onthe bearing (100) for sensing the positional change from the center ofthe rotating shaft (103), a controller (200) controlling the currentapplied to the magnetic bearing considering the rotating speed and theposition of the rotating shaft, and an amplifier (220) for amplifyingthe current to be applied to the magnetic bearing.

The controller (200) adjusts the magnitude of the current applied to themagnetic bearing (101) controlling the amplifier (220). Also, itcontinuously senses the current having been applied to the magneticbearing from the amplifier (220).

The vibration analyzer (212) senses the amplitude of the rotating shaft(103) of the bearing (100′) shown from the front. This can be a FFTanalyzer or an oscilloscope.

FIG. 9 is a flow chart illustrating a control process of the magneticbearing of the control system of FIG. 8. As illustrated, the controller(200) operates the magnetic bearing (101) at initial driving state, atexternal force application, or at resonant area of a bearing vibrationmode without current application to the magnetic bearing (101) (thefirst vibration mode).

At first, the magnetic bearing (101) is operated at initial drivingstate of a rotation device (S10) since the airfoil bearing (102) hardlygenerates dynamic pressure enough to support the rotating shaft at theinitial driving state. At W1, the airfoil bearing (102) generates enoughdynamic pressure to levitate the rotating shaft. If the rotating speed Wsensed by the sensor (211) is lager than W1, then the magnetic bearing(101) will be stopped (S20).

On the other hand, if the rotation device is stopped, then the magneticbearing (101) will be operated at the rotating speed of the rotatingshaft (103) between the levitation speed W1 and the zero speed. Sine thehybrid bearing of this embodiment is used for super high speed rotationdevice, the rotating speed cannot be under the levitation speed duringthe rotation device driving. Thus, the magnetic bearing should beoperated when the rotating speed W of the rotating shaft (103) is underthe levitation speed W1 (the airfoil bearing (101) levitates therotating shaft (103) at the levitation speed) after the rotation devicestopped (S30). If the rotating shaft has been completely stopped and therotating speed W is at zero, then the magnetic bearing (101) will bestopped (S31).

If a hybrid bearing of the present invention uses a low speed rotationdevice, then the magnetic bearing will be operated while the rotatingspeed W is below the levitation speed W1 until the rotating speed Wbecomes zero. Whether the rotating speed W is below the levitation speedW1 is sensed in real time without relation to whether the rotationdevice has been stopped.

The critical speed W2 is a resonant speed at the vibration mode withonly airfoil bearing of the airfoil-magnetic hybrid bearing operation.W0 is a 10% value of W2. In this embodiment, the magnetic bearing (101)operates while the rotating speed of the rotating shaft (103) is between90% critical speed and 110% critical speed. The controller estimateswhether the rotating speed W is between W2−W0 (90% critical speed) andW2+W0 (110% critical speed) (S40). If the estimated value is within therange, then the controller will operate the magnetic bearing (103)(S41).

If an external force is applied in the state over the levitation speedout of the resonant area, the rotating shaft (103) can vibrate. In thiscase, it can make a friction to the airfoil bearing, even can make afracture of the bearing or the rotating shaft. Thus, if the position ofthe center of the rotating shaft (103) gets out of the predeterminedamplitude, it will be preferred to operate the magnetic bearing (101). Srepresents predetermined amplitude for estimating external forceapplication. The controller (200) determines whether the displacement ofthe rotating shaft X sensed by a position sensor and analyzed in thevibration analyzer (212) exceeds the S (S50). If the displacement of therotating shaft (103) X is over S, then the controller will operate themagnetic bearing (101) (S51).

Referring to FIGS. 10 to 13, function and effects of the control systemfor the airfoil-magnetic hybrid bearing according to the presentinvention will now be described.

FIG. 10 illustrates orbit graphs of the rotating shafts, one rotatingshaft being supported by the airfoil-magnetic hybrid bearing while theother being supported by only an airfoil bearing. In FIG. 10, a boldline A indicates a displacement of the rotating shaft while the rotatingshaft (103) is supported by the airfoil-magnetic hybrid bearing. On theother hand, a fine line B indicates a displacement of rotating shaftwhile the rotating shaft is supported only by the airfoil bearing.

As illustrated, the supporting force of B is smaller since it hasinferior damping performance at the same RPM. Thus, the displacement ofthe rotating shaft is larger. Particularly, at 20000 RPM (resonantspeed), the displacement becomes very large. However the displacement ofA is very small. That is, the rotating shaft may not be far away fromthe center in A. Thus, it is understood that the damping performancebecomes considerably improved.

FIG. 11 is a graph illustrating controlled amplitude in the resonantarea using the airfoil-magnetic hybrid bearing according to the presentinvention. The hybrid bearing of the present invention has two vibrationmodes. Only airfoil bearing (102) is driven in the first vibration modeC, without operation of the magnetic bearing (101), while both of theairfoil bearing (102) and the magnetic bearing (101) are driven in thesecond vibration mode D.

As illustrated, the amplitude of the rotating shaft (103) follows theline C due to the increment of rotating speed after the rotation start.The amplitude increases significantly near the resonant area of thefirst vibration mode. At this time, the amplitude can be decreased bythe operation of the magnetic bearing (101). With the magnetic bearing(101) operation, the hybrid bearing follows the line D while theamplitude increases again due to the increment of the rotating speed.The amplitude in the vibration mode of line C decreases after resonance.Thus, the operation of the magnetic bearing should be stopped line C andline D cross. After line C and D cross, the vibration mode follows lineC. Thus, the amplitude of the rotating shaft (103) follows the bold linemaintained within predetermined amplitude. In this embodiment, themagnetic bearing (101) operates in the range between 90% resonant speedand 110% resonant speed of vibration mode of line C.

FIGS. 12 and 13 are graphs for comparing a waterfall of vibration of theairfoil-magnetic hybrid bearing according to the present invention to awaterfall of vibration of a prior art airfoil bearing. As illustrated,if only the airfoil bearing is operated, then the asynchronous vibrationcomponent indicated in the circle at the free end or the turbine end ofthe rotating shaft will be outstanding. However, it is understood thatthe vibration control performance can be improved since the aboveairfoil-magnetic hybrid bearing is used for additional damping of theasynchronous vibration component.

The bearing described in the above embodiments and accompanying drawingsis a journal bearing. However, the hybrid bearing can be adapted to athrust bearing supporting the rotating shaft axially. That is, aferromagnetic thrust disk is formed at the rotating shaft radially whilea hybrid bearing having an airfoil bearing and a magnetic bearing ismounted on a front and rear portion to the disk for supporting therotating shaft axially.

While the present invention has been described and illustrated withrespect to a preferred embodiment of the invention, it will be apparentto those skilled in the art that variations and modifications arepossible without deviating from the broad principles and teachings ofthe present invention, which should not be limited solely by the scopeof the claims appended hereto.

The invention claimed is:
 1. An airfoil-magnetic hybrid bearingcomprising: a housing around a rotating shaft of a rotation device; anairfoil bearing positioned between the housing and the rotating shaftand configured to form dynamic pressure to levitate the rotating shaftduring rotation of the rotating shaft; a magnetic bearing comprisingcores positioned radially between an inner surface of the housing and anouter surface of the airfoil bearing, the cores being projected to thecenter of the rotating shaft, and coils wound around the cores, thecoils configured to levitate the rotating shaft using a magneticattractive force generated by a current application thereto; and acoupling segment configured to couple the airfoil bearing to themagnetic bearing directly, wherein the spaces between the cores and thecoils are charged by dielectric bodies, wherein a fixing slot is formedat one of the dielectric bodies, wherein one end of the airfoil bearingis coupled to the coupling segment, wherein the fixing slot becomesnarrower while it becomes closer to the center of the rotating shaft,and wherein the coupling segment has a shape corresponding to the fixingslot, the coupling segment being inserted into the fixing slot in alongitudinal direction to the rotating shaft for fastening.
 2. Thehybrid bearing of claim 1, wherein the magnetic bearing operates inresonant speed area of a vibration mode without the magnetic bearingoperation.
 3. The hybrid bearing of claim 1, wherein the magneticbearing operates between an initial drive of the rotating shaft and alevitation speed of the rotating shaft while the rotating shaft islevitated by the airfoil bearing at the levitation speed.
 4. The hybridbearing of claim 1, wherein the magnetic bearing operates when thevibration displacement of the rotating shaft is bigger than thepredetermined amplitude.
 5. The hybrid bearing of any one of claims 1 to4, wherein the airfoil bearing includes a porous foil formed by a porouselastic body and a top foil laminated to the porous foil adjacent to therotating shaft, and wherein one end of the airfoil bearing is fixed tothe coupling segment and another end of the airfoil bearing is leftfree.
 6. The hybrid bearing of claim 1, wherein each one of the cores ispositioned radially between an inner surface of the housing and an outersurface of the airfoil bearing, each of the cores is projected to thecenter of the rotating shaft, and coils are wound around each of thecores.
 7. The hybrid bearing of claim 1, wherein the coils have adifferent winding density between an outside portion of the coil and aninside portion of the coil located closer to the rotating shaft than theoutside portion of the coil.
 8. The hybrid bearing of claim 7, wherein anumber of coil turns on an outside portion of the coil are equal to anumber of coil turns on an inside portion of the coil.
 9. The hybridbearing of claim 1, wherein the airfoil bearing is configured to becoupled to the magnetic bearing by the coupling segment while theairfoil bearing is disassociated from the housing to reduce a totalbearing volume.